1. Field of the Invention
The invention relates to a charge pump type of voltage booster circuit with a controlled number of stages. It is useful in any integrated circuit in which it is sought to produce a DC voltage of high value from a supply voltage. The invention is useful for example in the field of circuits comprising an electrically programmable memory.
A charge pump type of voltage booster circuit is used to produce a voltage within a circuit from a supply voltage received from outside, by transferring charges between consecutive capacitors so that the voltage produced (positive or negative) is greater in terms of absolute value than the supply voltage.
This type of circuit is used chiefly to supply capacitive circuits. For example, circuits of this kind are used in the field of electrically programmable volatile memories using metal-oxide-semiconductor ("MOS") type floating-gate transistors as storage cells, to produce programming and/or erasure voltages. The programming and erasure of these memories indeed requires voltage levels (of about 10 to 20 volts for example) far greater than that of the supply voltage generally used in integrated circuits (typically in the range of 3 to 5 volts).
2. Description of the Prior Art
FIG. 1 shows a known circuit, of the charge pump type (hereinafter called a charge pump) used to supply a capacitive type of circuit with a positive output voltage Vout produced from a supply voltage VCC. A circuit of this kind is described in the European patent 0 591 022 B1 owned by SGS-THOMSON Microelectronics S.A.
The schematic diagram of the pump is shown in FIG. 1. It has a set of n (with n as an integer) elementary stages C1 to Cn whose structure is illustrated in FIG. 2. The stages are series-connected between an input E and an output S and receive driving signals FX, FBX, FN, FBN called phases (illustrated in the timing diagrams 3a to 3d). The capacitive circuit is illustrated schematically in FIG. 1 by means of an equivalent capacitance Cout connected to the output S of the pump. It will be noted that the capacitance Cout could be the capacitance of an output capacitor that is physically present and integrated with the charge pump. The input E receives the voltage VCC.
An elementary stage 1, as shown in FIG. 2, comprises:
a first input 2 and a second input 3, PA1 a first output 4 and a second output 5, PA1 two synchronization inputs 6 and 7 to receive two clock signals CKL1 and CKL2, PA1 a first capacitor 8 connected between the inputs 2 and 6, PA1 a second capacitor 9 connected between the inputs 3 and 7, PA1 a first N channel MOS type transistor 10, its drain being connected to the input 2, its source being connected to the input 3 and its control gate being connected to the output 5, and PA1 a second N channel MOS type transistor 11, its drain being connected to the output 4, its source being connected to the input 2 and its control gate being connected to the input 3.
In the charge pump shown in FIG. 1, the stages C2 to Cn have their inputs 2 and 3 connected respectively to the outputs 4 and 5 of the previous stages C1 to Cn-1. The input 2 of the first stage C1 is connected to the input E by means of an N channel MOS type transistor TD1 mounted as a diode and an N channel MOS type transistor TE. The control gate of the transistor TE receives one of the phases (FBX in the example shown) and makes it possible for VCC to be supplied directly to the input 2 of the stage C1. The input 3 of the first stage C1 is not connected. The output 4 of the last stage Cn is connected to the output S. Its output 5 is connected to its output 4 by means of a N channel MOS type transistor TD2 mounted as a diode. This output 5 is connected furthermore to the first pole of a capacitor OC whose second pole receives one of the phases (FBN in the example shown).
The charge pump illustrated in FIG. 1 is driven by two pairs of phases: FN and FX on the one hand switching between two voltage levels, 0 volts and VCC, these phases being complementary but not overlapping in the high stage; FBN and FBX on the other hand respectively synchronized with the phase FN and FX but switching between two different voltage levels, 0 volts and Vb, where Vb is a voltage level greater by at least Vt (Vt being the threshold voltage, in taking account of the substrate effect, of the transistors 11 of the stages) than the voltage that must be let through by the transistors 11 of the stages of the charge pump.
The stages receive the phases FN and FBN or the phases FX and FBX at their inputs 6 and 7, two consecutive stages receiving two distinct pairs of phases. The transistor TE (and respectively the second pole of the capacitor OC) receives the phase FBX or the phase FBN depending on whether the first stage (and the last stage respectively) receives the phase FBN or the phase FBX.
With n series-connected stages, it is possible in theory to produce an output voltage Vout=(n+1)*VCC available at the output 4 of the last stage Cn by successive transfers of charges into the stages.
In practice, the value of (n+1)*VCC is reached only asymptotically at the output of the charge pump, i.e., the output voltage Vout increases at a constantly lower speed as and when charges are transferred from one stage to the other. It therefore often becomes necessary, in order to reduce the build-up time of the output voltage produced, to use a number of stages greater than the theoretically sufficient number. As it is generally preferred to produce an output voltage whose value is fixed, it becomes necessary to limit the voltage produced at the output of the pump to this value. This limiting can be done by stopping the transfer of the charges once the desired value is achieved, for example by dictating a constant potential at the level of the phases. It may also be done through the consumption, at the output of the charge pump, of the surplus charges produced. Typically, this consumption is obtained by means of a highly resistive arm whose conduction threshold (which of course corresponds to the desired value) is determined by the sum of the threshold voltages of series-connected diodes. This approach is generally not used as it induces a greater consumption of current. Indeed, the charge pump then works continuously.
Furthermore, it would be valuable to use a large number of stages if we consider the maximum current that can be given at the output of the pump for a given value of output voltage.
Let us consider for example the circuit of FIG. 4. It sets up a charge pump model supplying a resistive load RL. The voltage at the terminals of the load RL is Vout and it is crossed by a current IL. The charge pump is represented by a voltage source giving a no-load voltage Vlim and having an output resistance Rout. For a charge pump with n stages, we have Vlim=(n+1)*VCC. It can be shown that the current IL given is proportional to the operating frequency of the charge pump and to the value of the pumping capacitors.
FIG. 5 illustrates the value of the output voltage Vout as a function of the current IL for two pumps with a different number of stages. The output voltage of a charge pump with p stages is referenced V(p) and the output voltage of a charge pump with q stages is referenced V(q) assuming that q&lt;p. With q stages, we have Vlim=Vlq=(q+1)*VCC and with p stages we have Vlim=Vlp=(p+1)*VCC&gt;Vlq.
If we assume a voltage level Vs smaller than Vlq, it is observed that the current Imp given by the charge pump with p stages is greater than the maximum current lmq given by the charge pump with q stages, and that this is so inasmuch as Vout is greater than VCC If it is desired to supply a major current, it is therefore necessary to use a large number of stages. On the contrary, it is noted that for a given variation in current, the voltage drop for the charge pump with q stages is smaller than the voltage drop for the charge pump with p stages. In other words, in dynamic operating mode, it is useful to use a smaller number of stages. Indeed, it can be shown that the build-up time of the output voltage is proportional to the square of the number of pumping stages.
A present trend is to produce circuits working for wide ranges of supply voltage (which can be supplied for example without distinction with 3 volts or 5 volts). Now, if a circuit of this kind necessitates the use of a charge pump, the value of the output voltage to be produced is generally dictated by the physical parameters of the circuit; namely independently of the value of the supply voltage. In such a case, it is necessary to provide for a number of stages corresponding to the least favorable scenario; namely in practice a number of stages that is sufficient if the value of the supply voltage is the minimum. By placing a regulation circuit (also commonly known as a regulating circuit or a regulator circuit) at the output of the charge pump, the value of the voltage produced is limited if the supply voltage is greater than the minimum value. This limit makes it possible not to have any excessive variation of the voltage produced, especially an excessive output voltage when the supply voltage is high, which could give rise to premature aging or malfunctioning in the circuits supplied by the charge pump.
The definition of a charge pump capable of working for a wide range of supply voltages has drawbacks related to the need to set the number of stages as a function of the minimum supply voltage.
The output impedance of the pump is substantially constant whatever the supply voltage. In practice, this impedance is conversely proportional to the value of the capacitances of the stages and to the working frequency (namely, to the frequency of the phases), and proportional to the number of stages. Now it may be sought to minimize this output impedance. As stated, charge pumps are generally used to supply voltage to the capacitive circuits. The ability to reach the desired output voltage swiftly is therefore of vital importance. A penalty is therefore suffered in the favorable cases (with high supply voltage) where it would be sufficient to have a small number of stages, since the time constant at the output of the charge pump increases with the number of stages. One approach is to provide for a low working frequency. However, in this case, the maximum current diminishes and the build-up time of the output voltage increases. It is also possible to plan to reduce the value of the pumping capacitors. In this case, the maximum current available is also reduced. In practice, it becomes necessary to make choices with respect to the application envisaged, and it is not easy to set the size of a charge pump as the number of parameters to be taken into account is great and the impact of the modifications of a parameter induces effects that are both positive and negative depending on the characteristic studied. Accordingly, there is a need for a voltage booster circuit which overcomes these problems.